1. Field of the Invention
The present invention concerns a medium-voltage circuit-breaker in which insulation is provided by a gas with good dielectric properties such as sulfur hexafluoride (SF6), this gas also extinguishing the arc which forms when the arc contacts of the switch move apart.
2. Description of the Prior Art
This type of circuit-breaker includes an interrupter chamber which contains the arc contacts, and the gas in which is heated by the arc when these contacts separate with the result that the gas pressure is increased. The gas expands and extinguishes the arc at the first zero current crossing.
Problems are encountered with such equipment:
When low currents are interrupted (for example, currents less than or equal to the nominal current in the circuit in which the circuit-breaker is incorporated) the pressure increase may be insufficient or excessive, depending on the dimensions of the interrupter chamber. If the chamber is large, the pressure increase is small and the extinction effect may be insufficient; if the chamber is small the pressure rise is high but the extinction time may be insufficient to be properly effective. PA1 When high currents (short-circuit currents, for example) are interrupted the pressure increase must not be too large, as this could risk damage to the interrupter chamber.
One proposal to overcome these problems (see in particular the documents DE - A - 3 727 802 and EP - A - 0 315 505) is to provide an interrupter chamber whose volume is variable according to the current to be interrupted. This is achieved by replacing the fixed contact usually found in the circuit-breaker with a semi-fixed contact coupled to a spring-loaded piston. The displacement of the piston varies according to the current to be interrupted and the interrupter chamber is correspondingly larger or smaller. To be more precise, the interrupter chamber volume remains constant for low currents; if the current reaches a particular threshold, an additional volume is added; for medium currents to be interrupted the interrupter chamber volume is increased by displacement of the piston against the spring In devices of this kind the gas is polluted by the arc, which is prejudicial to effective extinction.
One aim of the present invention is to provide a circuit-breaker which does not suffer from this disadvantage and which, when a higher current threshold is reached, allows circulation of the gas in such a way as to improve the regeneration of its dielectric properties and to enable extinction at each root of the arc.